Oblique Hanle Measurements of InAs/GaAs Quantum Dot Spin-Light Emitting Diodes

Abstract

We report on studies of electrical spin injection from ferromagnetic Fe contacts into semiconductor light emitting diodes containing single layers of InAs/ GaAs self-assembled quantum dots ͑QDs͒. An oblique magnetic field is used to manipulate the spin of the injected electrons in the semiconductor. This approach allows us to measure the injected steady-state spin polarization in the QDs, P spin as well as estimate the spin losses in the QD spin detector. After subtraction of magneto-optical effects not related to spin injection, we measured a P spin of 7.5% at 15 K and estimated an injected spin polarization before QD recombination of around 20%. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2163074͔ Electrical injection of spin polarized carriers from a magnetic contact into a semiconductor is essential for the implementation of spintronic devices.1,2 An accurate way to quantify the spin injection process is to utilize a spin-light emitting diode ͑spin-LED͒. Such a device uses a magnetic contact to electrically inject spin polarized carriers into a LED structure and employs the radiative selection rules to relate the degree of the circular polarization of the emitted light to the injected carrier spin polarization. Ferromagnetic metals are attractive as magnetic contacts in spin-LEDs due to their high Curie temperatures and their well-studied structural and magnetic properties. Recently, high spin injection efficiencies have been achieved in devices where a conventional ferromagnetic metal such as Fe, Co, Ni and their alloys were used to electrically inject spin-polarized electrons via a Schottky 3-5 or an oxide tunnel barrier 6-9 into a semiconductor LED structure.In the great majority of these devices, bulk semiconductors or quantum wells have been used as the recombination region. Spin relaxation in these is mainly caused by momentum-dependent spin interactions that are in general fast because of the availability of a continuum of energy states. These mechanisms should be inefficient in spin-LEDs based on quantum dots ͑QDs͒ due to their three-dimensional carrier confinement, which results in a discrete density of states. Yet, there have been few reports on electrical spin injection in QD-based spin-LEDs, 10-12 only one of which 12 used a ferromagnetic contact as injector. In the latter, a large magnetic field ͑B Ͼ 2 T͒ was used to bring the Fe film magnetization out of plane ͑hard axis͒ and inject electrons with spins along the surface normal ͑Faraday geometry͒. An injected spin polarization in the QDs, P spin of 5% was measured; importantly the polarization was insensitive to the temperature and persisted to 300 K, confirming the potential of quantum dots as spin detectors in spin-LEDs. We demonstrate here electrical spin injection from ferromagnetic Fe contacts into semiconductor LED structures containing single layers of InAs/ GaAs self-assembled quantum dots ͑QDs͒. The spins are injected along the sample layers ͑Fe easy axis͒ under application of a small oblique magnetic field ͑B Ͻ 100 mT͒. The ...

Polarization Responses of a Solitary and Optically Injected Vertical Cavity Spin Laser

“…Such benefits are predicted for fairly modest spin-polarised pumping (S3 pump = 0.25 and 0.38 figure 9) and γ s values consistent with current materials at room temperature. Values of this order may be achievable with spin-aligning electrical contacts [2], [48] for future applications.…”

“…Note the change of sign for equation (A3) compared to equation (6.1-9d) in [48] due to the circular polarisation convention used (here φ = −, +π/2 for RCP,LCP, φ = +, −π/2 for RCP,LCP in [49]). Using the Stokes parameters from (A1) -(A3), ψ can be generated using equation (5).…”

LUMINESCENCE ENHANCEMENT OF OLED PERFORMANCE BY DOPING COLLOIDAL MAGNETIC FE₃O₄ NANOPARTICLES

“…It is known that recombination, covered by the fraction of excitons, which are as singlets, is limited to 25%, in case the created excitons ratio of triplet: singlet is 3 : 1 [45]. MNPs doped to emissive layer effect the spin states which resulting in an increase of the singlet exciton fraction and thus recombination process enhances [35,46].…”

“…On the other hand, the change in resistance more than 10% was achieved in polyuorene (PFO) devices at room temperature under weak magnetic field of 10 mT [33,34]. The effect of CoPt ferromagnetic nanowires on the exciton formation in poly (2-methoxy, 5-(2-ethylhexiloxy)1, 4-phenylenevinylene) (MEH-PPV) and iridium complex Ir(ppy)3 was investigated by Hu et al [35]. They reported that the spin-polarized holes transferred from CoPt nanowires to the polymer and iridium complex enhance the singlet to triplet ratio in the presence of magnetic field [35].…”

“…The effect of CoPt ferromagnetic nanowires on the exciton formation in poly (2-methoxy, 5-(2-ethylhexiloxy)1, 4-phenylenevinylene) (MEH-PPV) and iridium complex Ir(ppy)3 was investigated by Hu et al [35]. They reported that the spin-polarized holes transferred from CoPt nanowires to the polymer and iridium complex enhance the singlet to triplet ratio in the presence of magnetic field [35]. Sun et al observed that the quantum efficiency increased from 27% to 32% by doping 0.1 wt% CoFe magnetic nanoparticles into polymer matrix [36].…”